In a study of the mechanism of avian pancreatic polypeptide (aPP) secretion, we have demonstrated that mechanical expansion of the crop will cause a small but significant increase in aPP secretion, a finding which suggests a neurogenic origin for this stimulus. In collaboration with Dr. G. Duke of the University of Minnesota, a chronic turkey preparation has been developed in which gut segments can be isolated. From these experiments, it appears that the presence of food in the ileum but not duodenum or proventriculus is a potent stimulus for aPP secretion. Studies are in progress in which electrical field stimulation has been applied to isolated chick pancreas to evoke aPP secretion. Efforts to identify a target organ for aPP using 125I-aPP have indicated the pancreas, gut, spleen and possibly liver have a high capacity to bind the labelled hormone and that this binding is reduced in the presence of an excess of non-labelled aPP. A study of PP from 3 species has shown that avian, bovine and canine PP undergo reversible dimerization above pH 6. All three hormones have about 50% of their amino acid residues in alpha-helical conformation and this property is not changed by association-dissociation. In the case of aPP the dimerization has been shown to be entropically driven and presumably involves the hydrophobic faces of the alpha-helical segment. Charge interactions are also present in the dimerization.